1. Field of the Invention
The present invention relates to a photographing lens assembly provided with a motor-driven focusing device of the type in which an optical system, especially a focusing optical system, is moved by a motor for focus adjustment.
2. Description of the Prior Art
There is widely known and used such a photographing lens assembly provided with a driving mechanism for automatic focus adjustment. The driving mechanism generally includes a motor the rotation of which is controlled by a focus deviation signal coming from a focus detector for detecting the existing deviation of the image plane from the focal plane of the camera. The deviation of the image plane on which an image of the object is now being formed is detected electrically by the focus detector. In response to the deviation signal issued from the detector, the rotation of the motor is controlled, and in link with the rotation of the motor the driving mechanism moves the focusing optical system.
In this type of photographing lens assembly, the motor-driven focus adjustment mechanism becomes inactive when the power source battery for the motor is consumed or when the object to be taken is a very unique one (such as flat scene or smoke) for which the detection by the focus detector becomes very difficult.
In order to accommodate the lens assembly also to such cases, the applicants of the present patent application has already proposed a photographing lens assembly provided with not only the above-mentioned motor-driven automatic focus adjustment mechanism but also a manually operable focus adjustment mechanism. The user can select any desired one of the automatic focusing mechanism and the manual focusing mechanism independently of each other by a change-over operation. This photographing lens assembly is disclosed in the specification of U.S. patent application Ser. No. 296,142 filed on Aug. 25, 1981.
In general the manual focusing mechanism of photographing lens assembly is constructed in such manner that the moving range of the focusing optical system manually driven is limited between two end points and any further movement of the focusing system beyond the end points can be inhibited by mechanical limiting means. One of the limit end points lies at the position corresponding to the in-focus position to an object at infinity and the other limit end point lies at the position corresponding to the in-focus position to an object at the minimum object distance.
For the motor-driven automatic focus adjustment mechanism also it is desirable that the moving range of the focusing optical system be limited similarly to the above. This may be attained, preferably, by cutting off the power supply to the motor to stop the rotation or by switching over the direction of current flow to the motor to reverse the rotation of the motor when the focusing optical system reaches either limit end of the moving range.
Therefore, in case of the above-mentioned type of photographing lens assembly provided with both of manual and mechanical focusing mechanisms, it is required to provide both of mechanical limiting means and electrical limiting means for limiting the movement of the focusing optical system. This brings about some difficult problems. In particular when the automatic driving mechanism is selected for focus adjustment, both of mechanical limiting means and electrical limiting means are actuated at the same time at each the limit end point of the moving range of the focusing optical system. Consequently, the focusing optical system and its related mechanism being driven by the motor are subjected to impact force by said mechanical limiting means. The impact force causes the focusing optical and its related mechanism to move back and makes it difficult to stop the focusing optical system at the limit end point.
A solution to the problem is disclosed in Japanese Application for Utility Model Patent laid-open No. 51905/1982. According to the solution, a braking means is provided to moderate the shock at stopping. The braking means comprises a leaf spring which is applied to the moving member related to the focusing optical system when the latter comes near the limit end of the moving range so that the moving member can be impacted against the mechanical stopper with a reduced shock.
However, this known solution has some drawbacks.
To further move the moving member against the pressing force of the leaf spring a great deal of energy is required. To maintain the proper operability for focus adjustment the pressing force by the braking leaf spring must be very precisely set and adjusted. In practice it is very difficult to always correctly stop the focusing optical system just at the end point of the moving area without any loss of the easy operability employing this solution.